The invention relates to electronic devices comprising an audio amplifier and to methods for controlling such electronic devices.
Such electronic devices are widely used nowadays. The signal generated by the amplifier is sent directly or through another amplifier to loudspeakers for the reproduction of sound.
A problem may occur when the working mode of the device changes because the energy supplied to the amplifier may vary in a uncontrolled way and the generated signal may consequently be disturbed. This would give rise to what is known as a pop sound in the loudspeakers.
Various solutions have been proposed to reduce the disturbances.
It was for instance disclosed in patent application EP 0 785 501 to use a TTL POWERGOOD signal generated by a dedicated power failure detector of a power supply described in U.S. Pat. No. 5,224,010: a delayed version of POWERGOOD is utilised to mute the audio amplifier during the power-up and POWERGOOD is used to mute the audio amplifier during the power down.
However, such a power failure detector is a costly element and cheap power supplies are not provided with any.
Another solution which has been used is described as prior art in patent application EP 0 862 265: a microprocessor mutes the audio amplifier through a specific pin during interrupt routines which are triggered upon detection of a decreasing supply voltage. As explained in the patent application, this method is too slow, as the muting action is only generated after the decrease in voltage which creates the disturbance.
EP 0 862 286 proposes another solution using a fly-back transformer. This solution cannot therefore be implemented in electronic devices where such a fly-back transformer is not provided. Moreover, the proposed solution still uses a Zener diode, which is an expensive component.
The present invention seeks a solution to reduce the cost of the electronic device while at the same time reducing further the disturbances generated by the audio amplifier.
The invention proposes an electronic device comprising
an audio amplifier with a mute pin;
a first micro-processor connected to the mute pin
a second micro-processor having a mode pin;
a muting circuit with an input connected to the mode pin and with an output connected to the mute pin.
Further possible features are the following
a delay part of the muting circuit has an input connected to the mode pin and an output connected to a standby pin of the audio amplifier;
the delay part comprises a RC network interposed between the mode pin and the standby pin;
the RC network is connected to the base of a transistor whose collector is connected to the standby pin through a resistor;
a capacitor is interposed between the mode pin and the base of a transistor;
the collector of said transistor is connected to the mute pin via a diode and a resitor in series
the output of the muting circuit and a control pin of the first microprocessor are input to an OR circuit connected to the mute pin.
The invention further proposes a method for controlling the electronic device, comprising the succesive steps of:
generating a signal representative of an ON operating mode of the device on the mode pin;
generating a signal representative of an OFF operating mode of the device on the mode pin;
transmitting the signal representative of an OFF operating mode of the device to the mute pin through the muting circuit;
muting the audio amplifier.
Another feature of this method is possibly that the electronic device further includes a power supply with at least a DC output and that the period of time for transmitting the signal representative of an OFF operating mode of the device to the mute pin through the muting circuit is shorter than time constant of the DC output.
The invention also proposes a method for controlling the electronic device, comprising the successive steps of:
generating a signal representative of an OFF operating mode of the device on the mode pin;
generating a signal representative of an ON operating mode of the device on the mode pin;
switching the audio amplifier to standby mode when the signal representative of the ON mode is received on the standby pin;
waiting for a given period of time;
switching the audio amplifier to normal mode.